Infinity VS. Depth of Field

[boku]

feeling a bit saddened by the demise of Didger

Huh?

[dazzajl]

I think the diferences we are having here come down to a simple difference in approach. I was simply trying to answer a question relating to how a lens will act when taking a photograph, where as you seem to be more interested in swaggering around quoting text book ideals and being insulting (which is quite amusing btw   ).

At some point the difference in focus position and true infinity becomes too small to measure, and people don't bother any more

Exactly, because it doesn't make any difference when you are taking the photograph. Going back the simple illustration of walking away from a camera and where you appear on the focus scale, at some point (and I dare not mention it by name) you cease to need to adjust the focus.

You can spout as much technical theory as you like but none of it will change the answer to the original question about where to focus when everything in the scene is futher away than the inf...... almost said it there.  :laugh:

[Gordon Buck]

Probably an old joke even then but I laughed and it still brings a smile  – perhaps because that was long ago (seems longer every day) and reminds me of those days ….

The professor wrote 8 / 0 = "infinity sign"  on the chalkboard and launched into a lecture on limits, defined expressions, undefined expressions, infinity, etc.  The class took it all in, or so he thought.  A few days later, he gave the class a test.  One of the problems on the test was:  5 / 0 = ?     Of course, one student gave the answer as  “5 on its side”  (you’ll have to use your imagination here since I can’t make a 5 in landscape orientation).

Thereafter, we always called the infinity symbol the “lazy eight”.

Well, it was funnier when the characters could be drawn on the chalkboard.

[boku]

I beg to differ. When I was in college during the late 60's, I definitely remember several instances when I went beyond infinity and came back a few days later. I rapped with Jimi when I was there.

Peace, brother.

[dazzajl]

Jonathan,

You seen to have a penchant for rather long and wordy posts, I'm going to try and keep this one short and simple so I hope it still makes sense to you.

Of course you can have "beyond infinty", when in the context of distance from a lens. Every lens has a point at which further distances from the lens are no longer relevant in terms of focus. This distance is called infinty. It is not a real infinity and hence may lead to some confusion.

Once you are focused on this infinty distance, anything beyond that (no matter how far) will be covered by the plane of focus. Hyperfocal distance has absolutley nothing to do with it.

In fact, if you want to get into the whole techy debate thing, there is no such thing as hyperfocal distance as there is really no such thing as depth of field.

D  

[Jonathan Wienke]

For temperature variations, many lenses have the ability to focus beyond infinity. If they can focus far enough beyond infinity, this can also help with certain lens adapters for those times you use otherwise incompatible lenses with your camera.

It is true that many lenses allow additional focus adjustment beyond the infinity mark for the reasons you mentioned. But that certainly does not mean that the actual subject distance is beyond infinity. It's simply an allowance for some variation in the focus ring position that actually corresponds to infinity focus.

[dazzajl]

but they are as sharp as they can be because they are in focus

Which essentially means we are saying much the same thing.

The problem here is that some people are talking about infinty as the concept of distance/number without measure and others about the distance represented by the symbol on a lens.

[Jonathan Wienke]

As you stated in an earlier post, it's all in the terminology. The terminology you've been using is made-up and non-standard. When a lens is focused at infinity, there will be a wide range of distances where objects will appear to be "in focus". Understanding how DOF is calculated (and hyperfocal distance in particular) will help you figure you how far away something has to be for it to be just as well-focused as a distant star, as well as when it is advantageous to set the focus distance to something less than infinity to get the widest possible range of distances in acceptably sharp focus, and what aperture one should use while doing so.

Understanding the underlying technical concepts and issues are critically important in photography. I make my living from photography and shoot over 50,000 frames per year. I'm not just spouting irrelevant technobabble because I can, I'm trying to educate you in the importance of correctly understanding the process and technology of photography (which can improve the quality of your work if accompanied with appropriate creativity and talent) and the correct terminology, so that you can have meaningful discussions with others about photography without looking and sounding foolish and ignorant. Knowing the technical stuff inside and out will make you a better photographer; don't neglect it.